As an important component part for reading out signals from optical recording media such as optical discs, e.g. compact discs (CD), DVD etc., optical card memories or the like, an optical head is available. In order to take out signals from the optical recording media, the optical head needs to be equipped not only with a signal detection function but also with a control mechanism such as a focusing servo, a tracking servo and the like.
The optical head usually includes various optical components such as a light source, a photodetector, an objective lens, a focus/tracking error signal detecting element, a mirror, a collimator lens etc. A laser beam emitted from a light source is focused on an optical disc by an objective lens (collective lens). The laser beam focused on the optical disc is reflected and detected by a photodetector. In this way, a reproduction signal is read out. Moreover, focusing and tracking are controlled by a focus/tracking error signal detecting element, so that signals can be read out stably.
However, in the case of a light source that has a wavelength broadening such as a semiconductor laser source to which high frequency current superposition is applied, due to chromatic aberration (in particular, mainly due to color dispersion of a glass material) of the optical system including the objective lens, defocusing of a focused spot on the information recording medium was likely to occur according to the degree of wavelength broadening.